Standard star gems are around 500K to make (not counting resell value of the components) so 1.5M + 80K = radient and then 3 X 1.6M + 100K (5M) for next up, and then 3 X 5M + 200K = slightly over 15M for top tier. I rounded up a little bit so it's probably awfully close to 15M even. Anybody selling for much less than 18M on the GAH is losing money after they take their 15% cut.